The prevention of undue or excess leakage currents in connection with refractory-encased electric resistance heater elements, such as sheathed tubular heating elements, for example, is a continuing area of concern in the electric appliance industry. Refractory-encased and especially metal-sheathed heater elements are widely used by the public and appear in diverse appliances including electric ranges, electric ovens, hot water heaters and other electrically heated devices.
While the manufacture of such heating elements varies in details of size, termination, etc. for particular applications, the same quite commonly employ an elongated electrical resistance member disposed centrally within and insulated by a compacted monolithic refractory received within an external rigid tubular metal sheath. The resistance member has terminal leads extending from both ends of a sheath length for connection to a supply of electrical energy. Magnesium oxide and like materials are commonly employed as the refractory into which the heating element is embedded.
It is, of course, essential that the electrical resistance element carrying the current be insulated from the surrounding metal sheath which in the instance of an oven or cooking range, for example, is readily accessible to a consumer or contact with a part of the body.
As a consequence, in manufacture, the resistance element is normally maintained substantially centrally or concentrically of the sheath by the refractory, commonly magnesium oxide or the like. While excellent insulation, such material is highly hygroscopic.
As a consequence, in a given environment or over a protracted period of time with ambient conditions of relatively high humidity, moisture in the form of water vapor is attracted to and tends to wick its way into the heater element, thereby promoting a leakage current or mild short circuit path between the end of the terminal and the surrounding sheath. Accordingly, should the moisture buildup become too great, there is a danger of a considerable current leakage between the terminal pin at either end and the surrounding sheath thereby providing significant current flow from the external otherwise cold heater sheath which can cause discomfort or electric shock to an individual accidently touching the same in working around a stove or in handling other appliances such as an electric iron, electric skillet, etc.
In those circuits, such as outdoor circuits, normally protected by ground fault interrupters, the unwanted leakage can build up to be sufficiently excessive to trip the breaker, thus causing unintended interruption or shutdown of service.